Recent years have seen the emergence of music delivery using high resolution sound sources, audio data that offer higher quality than music CDs (CD-DAs). Lossless compression technologies such as FLAC (Free Lossless Audio Codec) are available for PCM (Pulse Code Modulation) sound sources including 96 kHz/24 bit and have found application for delivery.
For a one-bit delta-sigma-modulated digital signal (DSD (Direct Stream Digital) data) rather than a PCM digital signal, on the other hand, a lossless compression technology called DST (Direct Stream Transfer) developed by Philips is available and used to create a Super Audio CD (SACD) disc.
However, this technology is based on one-bit signal processing and is not fit for software processing using a CPU that is based on byte-by-byte processing. Therefore, this technology is implemented in hardware (LSI) in SACD players and so on. It is difficult for an ordinary embedded CPU to handle processing using software due to a heavy processing load.
Therefore, in the case where audio signals are delivered using DSD data, a lossless compression technology is required that allows even an ordinary embedded CPU to handle processing in consideration of execution by a mobile terminal.
The present applicant has proposed, as an audio signal lossless compression technology using DSD data, a technology for referring to past data, four bits by four bits, and compressing current data into two bits in PTL 1.